The general operating systems nowadays support a disk snapshot technique which is also called volume shadow copy services. Being different from general data backup techniques which relate to file copying, the so-called disk snapshot technique preserves the file system and backs up the volume (the volume refers to the disk chunks containing data in a hard disk) of a hard disk anytime the users desire. When performing disk snapshot, there is no file copying operation and the directory structure of the file system in use will be preserved completely. Hence, the disk snapshot technique is able to establish volumes of a hard disk of different time versions, thus improve the conventional backup method. Besides, when a hard disk got damaged or infected, the administrator may replace the original volume by the disk snapshots established in advance to minimize the loss.
As stated above, there is no file copying when performing disk snapshot. Instead, when the disk chunk of the volume is changed, the original data of the disk chunk of the volume is copied to other available disk chuck which does not belong to the volume, then the original data of the disk chunk of the volume is modified into modification data, at the same time a Copy-On-Write table (hereinafter abbreviated as “COW table”) is created in a memory space used by the internal kernel of the operating system (for example: Linux) provided by the memory of the server. The COW table records the information of which disk chunk of the hard disk is the original data of each disk chunk stored. The technique stated above is called the Copy-On-Write technique.
However, the memory space of the operating system kernel is limited, and will not increase as the system memory getting larger. When the change of the disk chunk of the volume increases, the memory space of the operating system kernel occupied by the COW table becomes larger. The internal kernel space will be occupied by a huge number of COW table when performing disk snapshot at different time or when the amount of the volume increases tremendously, results in collapse of the operating system. As a result, there exists urgent need of other disk snapshot method to solve the problems stated above.